The present invention relates to a process for reactivating a spent dehydrocyclodimerization catalyst, thereby extending the useful life of the catalyst.
Dehydrocyclodimerization is a process in which aliphatic hydrocarbons containing from 2 to 6 carbon atoms per molecule are reacted over a catalyst to produce a high yield of aromatics and hydrogen, with a light ends byproduct and a C2-C4 recycle product. This process is well known and is described in detail in U.S. Pat. Nos. 4,654,455 and 4,746,763 which are incorporated by reference. Typically, the dehydrocyclodimerization reaction is carried out at temperatures in excess of 500xc2x0 C. (932xc2x0 F.), using dual functional catalysts containing acidic and dehydrogenation components. The acidic function is usually provided by a zeolite which promotes the oligomerization and aromatization reactions, while a non-noble metal component promotes the dehydrogenation function. One specific example of a catalyst disclosed in U.S. Pat. No. 4,746,763 consists of a ZSM-5 type zeolite, gallium and a phosphorus containing alumina as a binder.
The conditions used for the dehydrocyclodimerization reaction result in catalyst deactivation which is believed to be caused by excessive carbon formation (coking) on the catalyst surface. After several days (from about 3 to 10 depending on the operating temperature) enough activity has been lost due to coke deposition that regeneration of the catalyst is necessary. Regeneration involves burning or oxidizing the coke present on the catalyst at elevated temperatures. In addition to loss of activity due to coke formation, catalysts containing a phosphorus modified alumina as a binder are gradually deactivated (over a period of time from several months to about a year) by exposure to hydrogen at temperatures generally greater than 500xc2x0 C. (932xc2x0 F.) and particularly greater than 565xc2x0 C. (1049xc2x0 F.). This loss of activity due to hydrogen exposure cannot be restored by regeneration means, i.e., burning or oxidation at elevated temperatures. Therefore, a process is sought for reactivating catalysts that have lost activity due to exposure to hydrogen above 500xc2x0 C. (932xc2x0 F.).
As used in this application, regeneration refers to the process of restoring lost activity due to coke formation, while reactivation refers to the process of restoring lost activity due to hydrogen exposure.
U.S. Pat. No. 5,212,127 (Kocal et al.) describes a process for restoring substantially all of the activity of a dehydrocyclodimerization catalyst that has been deactivated by exposure to hydrogen at high temperature. This patent teaches treating the catalyst with an aqueous solution of a weakly acidic ammonium salt or a dilute acid solution, rinsing the treated catalyst, and oxidizing the rinsed catalyst to produce reactivated catalyst. The treating solution should contain from about 0.1 to about 5 moles/liter of the salt or acid, and the preferred treating solution is an ammonium nitrate solution.
One of the problems with the reactivation process in U.S. Pat. No. 5,212,127 is the use of the salt or the acid in the treating solution. Expenses arise as a result of purchasing, handling and ultimately disposing of the salt or the acid. In addition, treating with the salt or the acid necessitates a rinsing step after treating and prior to oxidizing. Therefore, a process is sought for reactivating catalysts that have lost activity due to exposure to hydrogen above 500xc2x0 C. (932xc2x0 F.), wherein the process does not require a salt or acid in the treating solution. Preferably, the process should be easily incorporated and employed in commercial catalytic dehydrocyclodimerization processes.
This invention provides a reactivation process in which the catalyst is treated with a fluid comprising water at a temperature and for a time sufficient to restore at least part of the activity of the catalyst. Thus, this invention does not require an aqueous solution of a weakly acidic ammonium salt or a dilute acid solution in order to reactivate the catalyst. Fluid water, by which it is meant water that is in a non-solid phase such as liquid water or water vapor (steam), at least partially reactivates the catalyst. After the water treatment, the catalyst is dried, such as by oxidizing in air, at a temperature and for a time sufficient to at least partially reactivate the catalyst.
The discovery that liquid water, rather than a solution of water and salts or acids, can be the washing solution in a process for reactivating hydrogen-deactivated catalysts provides part of the basis for this invention. In addition, it has been discovered that water vapor (steam) can also be a reactivating agent in a process for reactivating hydrogen-deactivated catalyst. Although this invention is not limited to any particular explanation or theory for this discovery, it is believed that high temperature hydrogen deactivates the catalyst by removing hydroxyl groups, and that the fluid comprising water replenishes these hydroxyl groups, regardless of the presence of salts or acids. Another possible explanation for why fluid water reactivates the catalyst is that the water removes some deleterious species that have formed on the catalyst. Although the water may be removing aluminum and phosphorus, the water may also be removing some sodium, calcium, and silicon, as evidenced by analysis of the water after treating a hydrogen deactivated catalyst with liquid water.
In another surprising aspect of this invention, it has been observed that the range of temperature and pressure conditions under which the liquid water reactivates the catalyst is relatively wide, in contrast to a much narrower range of temperature and pressure conditions under which steam reactivates the catalyst. Furthermore, it has been discovered that steam can reactivate the catalyst, not only when pure steam is used but also when a mixture of steam and a diluent such as air is used. It has also been discovered that steam is more effective as a reactivating agent under steaming conditions when the steaming temperature is relatively low and the steam content is relatively high (i.e., little or no diluent.) One theory that may explain why these steaming conditions are particularly and unexpectedly effective, as well as why liquid water is effective as a reactivating agent, is that the reactivation mechanism may involve wetting of the surfaces or pores of the catalyst with liquid water, and that such wetting is more likely to occur, even though water vapor (steam) is the reactivating agent, when the steaming conditions are closer to the point of saturation. Whatever the explanation, the conditions under which steam reactivates the catalyst are particularly advantageous because they allow this invention to be easily incorporated and employed in commercial catalytic dehydrocyclodimerization processes.
Accordingly, the instant invention relates to a process for reactivating a dehydrocyclodimerization catalyst that has been deactivated by exposure to hydrogen at a temperature above 500xc2x0 C. (932xc2x0 F.). The process comprises contacting the catalyst with fluid water to produce a treated catalyst and then drying the treated catalyst, thereby providing a reactivated catalyst. The concentration of an acidic ammonium salt or of an acid in the fluid water is less than about 0.1 mole/liter.
In another of its aspects, this invention also relates to a method of producing a catalyst that is resistant to deactivation by exposure to hydrogen at a temperature above 500xc2x0 C. (932xc2x0 F.). The method of producing the catalyst comprises a step in which the catalyst is treated with a dilute acid solution which has an acid concentration that is less than about 0.1 moles/liter. In the prior art, this treatment step required an acid concentration of at least 0.1 moles/liter.